The present invention generally relates to the art of digital imaging. It finds particular application in conjunction with estimating local defocus distance and correcting geometric distortion in a scanned image of a bound book, and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with correction of geometric distortions in other three-dimensional objects on a variety of image acquisition and output devices, such as scanners, xerographic copiers and printers.
Optical systems in document scanners, such as a Selfoc(copyright) lens scanner or xerographic apparatus, possess a depth of field over which the optical system produces an image that meets resolution and distortion specifications. Conventionally, when a book or other bound, multi-page original is scanned on the scanner, the resulting digital image contains geometric distortions and blurring in the region adjacent the binding. In other words, portions of certain non-planar original objects, such as books, wrinkled sheets, and three-dimensional objects, may lie outside the depth of field of the imaging system. For these originals, the distance between the original object and the scanner platen surface, i.e. the ideal object plane, varies from point to point, which may cause a spatially varying blur in the acquired image. In addition to blurring, geometric distortion and illumination falloff often occur due to the varying defocus distance.
The above geometric distortion problem has been addressed in a variety of ways in the prior art. According to one prior art method, many scans are acquired at fine digital resolutions at a variety of imaging depths. In processing the image, the image values are sub-sampled in order to retain only those samples acquired at the optimal focus. However, this method requires a greater number of scans, more data to be stored in memory, and greater computational complexity.
Another solution to the above problem is to modify the platen to have a sloping edge portion whereby the bound portion of the book is placed in a corner position such that the entire surface of the page being copied is in close contact with the platen surface. This system suffers from a limited magnification range because of restriction on scanned component movement near the sloping corner edge. In addition, operability and production are limited by the inability to perform a xe2x80x9csplit scan,xe2x80x9d where both pages of a book are scanned without repositioning.
Another solution to the geometric distortion problem employs an infrared distance sensor to determine the object defocus distance. In this method, the plurality of sensed object defocus distances are used in a subsequent digital restoration algorithm. Another prior art technique uses one or more standard reference targets in order to determine the defocus or degraded states of specific scanner systems. The scanned image is then filtered with a fixed or time-varying image restoration filter, which utilizes correction coefficients pre-selected for the state of defocus or degradation. In addition, profile detectors have been inserted into the scanning system in order to measure the defocus distance of an object by using a reference image plane of the scanning system. The above prior art techniques suffer from the disadvantage of additional required component costs as well as unwanted image processing complexity.
Therefore, a need exists for an accurate method of estimating defocus distance without the use of additional hardware. The present invention contemplates a new and improved method of determining defocus distance, and therefore correcting geometric distortions using a single scanned image of the three-dimensional object acquired in a conventional document scanning manner, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of determining local defocus distance in a scanned image of a non-planar original object includes scanning at least a portion of the non-planar original object to produce scanned image data. An image feature that is obtainable from the scanned image data and a quantitative metric, which corresponds to the image feature, are selected. A calibration relationship is established between the selected image feature and the defocus distance of the non-planar original object. Image feature data is extracted from the scanned image data and used in the calculation of a plurality of defocus distances from the extracted image feature data. The plurality of defocus distances is organized into a defocus distance map.
In accordance with a more limited aspect of the present invention, edges of text within the scanned image are selected as the image feature and the slope of the edges of text are selected as the corresponding quantitative metric.
In accordance with a more limited aspect of the present invention, the step of calculating a plurality of defocus distances includes calculating slope values for the detected text edges and averaging the calculated slope values along a first direction to generate an average slope value. The plurality of defocus distances are then calculated based upon the average slope values and the calibration relationship.
In accordance with another aspect of the present invention, a digital imaging method for imaging an open book having a book binding includes scanning the open book to produce scanned image data, where the scanned image data contains at least one of geometric distortions and blurring defects adjacent the book binding. Local defocus distances are determined at a plurality of positions adjacent the book binding from at least one image feature obtainable from the scanned image data. The local defocus distances are organized into a defocus map and a distortion map is constructed from the defocus map. The scanned image data is corrected in accordance with the geometric distortion map and outputted onto a physical media.
In accordance with a more limited aspect of the present invention, the step of determining local defocus distance includes selecting an image feature obtainable from the scanned image data and a quantitative metric corresponding to the selected image feature. A calibration relationship is established between the selected image feature and the defocus distance at a plurality of positions across the scanned image. Image feature data is extracted from the scanned image data and used along with the calibration relationship to calculate the local defocus distance at a plurality of positions adjacent the binding of the book.
In accordance with a more limited aspect of the present invention, the selected image feature includes edges of text within the scanned image and the quantitative metric includes a slope of the edges of text within the scanned image.
In accordance with another aspect of the present invention, a xerographic apparatus for reproducing an image representation of a scanned open book includes a planar imaging platen and means for scanning the open book to produce scanned image data, which contains at least one of geometric distortions, blurring defects, and illumination variations adjacent the binding of the book. A processor calculates a defocus distance map from image feature data extracted from the scanned image data by appropriate means. A processor calculates a distortion map from the defocus distance map and an image processor corrects the scanned image data for at least one of the geometric distortions, blurring defects, and illumination variations. The apparatus includes means for transferring an image charge pattern onto a photoreceptor. At least one developing station develops the charge pattern and at least one fuser station fixes the developed image onto a physical media.
In accordance with a more limited aspect of the present invention, the apparatus includes an edge detection processor which detects edges of text within the scanned open book.
In accordance with a more limited aspect of the present invention, the processor for calculating the defocus distance map includes means for calculating a slope corresponding to each detected text edge and means for calculating an average slope from each calculated slope along a direction parallel to the binding of the book. The processor further includes means for accessing a predetermined calibration relationship between slopes of text edges and corresponding defocus distances and means for calculating a defocus distance from the average slope and the predetermined calibration relationship.
One advantage of the present invention resides in determining defocus distance from a single scanned image of a three-dimensional object.
Another advantage of the present invention resides in the determination of local defocus distance using reference features within a scanned image.
Another advantage of the present invention resides in improved scanning of a bound object without geometric distortion and blurring adjacent the binding.
Still other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.